The present invention relates to a print head assembly for printing a print medium, such as a fluid ink or a solder paste, onto workpieces, including flexible or delicate substrates, for example, as employed in solar or fuel cell applications, through a printing screen which includes a pattern of printing apertures corresponding to a pattern of deposits to be printed on a workpiece, a screen printing system incorporating the same, and a screen printing method.
Very many screen printing systems and print heads have been developed for the printing of print media onto workpieces.
FIGS. 1 and 2 illustrate a typical screen printing system which utilizes a print head in the form of a squeegee.
The screen printing system comprises a rigid support frame 3, which supports a printing screen 5, a pair of spaced, parallel linear bearings 7, 7 which are mounted to the support frame 3 at opposite sides of the printing screen 5, a print carriage 9 which includes a gantry 11 which is supported on the linear bearings 7, 7, and a print head assembly 15 which is mounted to the gantry 11 and operable to print a print medium through a pattern of printing apertures in the printing screen 5 onto a workpiece which is supported therebelow, and a drive unit 17, which comprises a drive motor 19 and a toothed drive belt 21 which is coupled to the print carriage 9, for reciprocatingly driving the print carriage 9 along the linear bearings 7, 7.
The print head assembly 15 comprises first and second squeegees 23, 25, which are juxtaposed in parallel relation and independently vertically movable in order to allow for configuration in dependence upon the direction of printing, and a drive mechanism 27 which couples the squeegees 23, 25 to the gantry 11 by a spring coupling (not illustrated) and is operable vertically to raise and lower the squeegees 23, 25 and maintain a downward force on the operative one of the squeegees 23, 25. The drive mechanism 27 comprises first and second actuators 31, 33, here stepper motors coupled by leadscrews, which are operable vertically to raise and lower respective ones of the squeegees 23, 25.
Operation of the print head assembly 15 is illustrated in FIGS. 2(a) and (b). As illustrated in FIG. 2(a), where printing is in a first direction D1 by the first squeegee 23, the first actuator 31 is operated to drive the first squeegee 23 downwardly into contact with the printing screen 5 and raise the drive mechanism 27 against the bias of the spring coupling, such that a predetermined downward biasing force is applied to the first squeegee 23, and the second actuator 33 is operated to drive the second squeegee 25 upwardly, such that the lower edge of the second squeegee 25 is raised clear of the face of the first squeegee 23 in order to allow for a clear space forwardly of the first squeegee 23 in the first direction of printing D1, which is necessary to allow for rolling of the print medium ahead of the first squeegee 23. Similarly, as illustrated in FIG. 2(b), where printing is in the other, second direction D2 by the second squeegee 25, the second actuator 33 is operated to drive the second squeegee 25 downwardly into contact with the printing screen 5 and raise the drive mechanism 27 against the bias of the spring coupling, such that a predetermined downward biasing force is applied to the second squeegee 25, and the first actuator 31 is operated to drive the first squeegee 23 upwardly, such that the lower edge of the first squeegee 23 is raised clear of the face of the second squeegee 25 in order to allow for a clear space forwardly of the second squeegee 25 in the second direction of printing D2, which is necessary to allow for rolling of the print medium ahead of the second squeegee 25.
This screen printing system has operated very successfully, but the present inventor has recognized that this screen printing system suffers from a number of drawbacks, particularly in relation to the printing of fluid inks onto flexible and delicate workpieces. In particular, the bulk of the gantry 11, which is necessary to confer the required rigidity, and the weight of the drive mechanism 27 which is mounted to gantry 11, limits the speed of movement of the print head assembly 15 and hence the machine throughput, and also induces vibrations at the start and end of each print stroke, which vibrations are particularly detrimental in relation to the printing of fluid inks onto flexible and delicate workpieces. In addition, the bulk of the gantry 11 restricts the view of the printing screen 5.
Enclosed print heads have also been developed. Examples include the applicant's earlier WO-A-1998/016387 and WO-A-2001/005592, where these screen printing heads are sold under the ProFlow® brand.
These enclosed print heads have operated very successfully to date, particularly in the printing of solder pastes onto printed circuit boards, but new applications, particularly in the production of solar and fuel cells, which utilize flexible and delicate substrates and require the printing of fluid inks, are presenting needs which cannot be met by these existing screen printing heads.